The present invention relates to an improved process for preparing acetophenones which are substituted on the aromatic ring by fluoroalkyl. Such compounds are useful intermediates for preparing antibacterial agents (see DE-A 26 04 207).
A known process for preparing trifluoromethylacetophenone on an industrial scale involves the preparation, from trifluoromethylaniline and sodium nitrite in the presence of hydrochloric acid, of the corresponding diazonium salt solution, which is then buffered with sodium acetate and added, at from 5 to 15.degree. C., to an initial charge comprising water, acetaldoxime, a copper(II) salt, a reducing agent (sodium thiosulphate) and a large amount of sodium acetate buffer. For work-up, hydrochloric acid is added and the mixture is heated to reflux, subjected to steam distillation and extracted, and the distillate is dried, the extracting agent is stripped off and the residue is distilled under reduced pressure. Based on the fluoromethylaniline employed, this gives trifluoromethylacetophenone in a yield of 33% of theory (see DE-A 26 04 207, Preparation Example 1.4).
In addition to the low yield, this process has the disadvantage that large amounts of auxiliaries, for example buffer salts, reducing agents and hydrochloric acid, are used which render work-up more difficult, result in a considerable salt load of the waste water and, after they have been separated off, cause great costs for an environmentally safe disposal.
Other known processes for preparing trifluoromethylacetophenone (see, for example, Liebigs Ann. Chem. 717, 80-90 (1968), J.Am. Chem. Soc. 68, 736 (1946), J. Organomet. Chem. 489, (1-2), 137-143 (1995)) are not suitable to be employed on an industrial scale because they require apparatus and reagents which are expensive and difficult to handle.